1. Technical Field of the Invention
The present invention relates, in general, to mobile communications, in particular, to a system and method for efficient time of arrival measurements of access bursts for mobile station positioning, and, more particularly, to a system and method for the efficient allocation and deallocation of communications resources such as control channels utilized in positioning measurements.
2. Description of Related Art
Mobile radio position determination is an emerging field that has gained a great deal of interest lately, and it is now desirable, as well as required by law, that companies include position determination capability in future cellular mobile radio systems. The Time Difference of Arrival (TDOA) or Time of Arrival (TOA) position determination method has been used successfully for determining the position of mobile radio terminals. A typical TDOA position determination system can be either terminal-based, whereby TOA measurements are made on the xe2x80x9cdownlinkxe2x80x9d in the mobile station (MS), or network-based, whereby the network performs the TOA measurements on the xe2x80x9cuplinkxe2x80x9d in the radio base transceiver stations (BTSs). These TOA measurements are then used to calculate TDOA parameters and estimate the MS""s position within the telecommunication system.
A network-based method for determining the position of cellular mobile stations is disclosed in commonly-assigned Swedish Patent Application No. 9303561-3 to R. Bodin. In order to determine the position of a mobile station, a handover procedure is initiated between a serving base transceiver station and the mobile station. The mobile station transmits access request signals to a new base transceiver station. The base transceiver station measures the time delay for the access request signal to travel between the mobile station and the base transceiver station. This procedure is repeated between the mobile station and one or more additional base transceiver stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base transceiver stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on so-called asynchronous handovers, where the target base transceiver stations measure the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base transceiver station. At least two positioning handover operations are needed to obtain three such distances, which can be used in a triangulation algorithm to determine the mobile terminal""s position. Notably, one distance can be obtained between the serving base transceiver station and the mobile terminal without a positioning handover. For example, in the Global System for Mobile Communications (GSM), the Timing Advance (TA) value used for time alignment of bursts can optionally be used as a representation of the distance in the serving cell. A more accurate position determination can be attained if more than two such positioning handovers are made, because more than three distances will be known. The use of more than three distance measurements compensates for some errors arising in the individual measurements.
Although the above-described document illustrates considerable progress in the cellular position determination field, there are still a number of deficiencies to be improved upon. For instance, in existing GSM systems having TOA-based mobile station positioning capabilities, the determination of a mobile station position is not highly accurate. The calculation of the number of access bursts for reception from a mobile station is based upon the type of application and/or the degree of accuracy specified. In addition, the identification of particular time measuring units for use in measuring the time of arrival of transmitted access burst signals is based upon only a general location of the mobile station, such as information gleaned from cell identification and timing advance (TA) parameters. As a result, mobile network resources may not be optimally utilized in determining mobile station position. Network resources in existing GSM systems may be overly utilized to determine mobile station position, thereby reducing the quality of other services provided by the telecommunications system. Alternatively, network resources may be under utilized, resulting in a less accurate mobile station position determination.
In particular, most recently it has been proposed to have Generic Measurement Units (GEMUs) measure the time of arrival of the access bursts received from the MS being positioned. In this positioning system, a GEMU is located in physical proximity to each BTS of the MS positioning capable network. The GEMU communicates with the Mobile services Switching Center/Visitor Location Register (MSC/VLR) according to the Direct Transfer Application Part (DTAP) of the Signaling System #7 on the Stand alone Dedicated Control Channel (SDCCH). The DTAP setups may be made either dynamically on demand or semi-permanently for each GEMU.
When the DTAP is allocated semi-permanently, however, the respective SDCCH channel is occupied whether or not there is an MS positioning calculation being performed. Thus, the occupied SDCCH channel is unavailable for other use, e.g. call setup, transmission of textual messages such as short message service (SMS) or cell broadcast messages, etc., when allocated semi-permanently even though a positioning calculation may not be currently performed. Consequently, there exists a need for an improved TOA-based, mobile station positioning system which alleviates this resource allocation problem, resulting in a more efficient telecommunications system.
It is, therefore, a first object of the present invention to provide an improved system and method for measuring time of arrival signals in a mobile communications network.
It is another object of the present invention to provide for better resource utilization in a mobile station positioning-capable mobile communications network.
It is yet another object of the present invention to provide improved utilization of the Stand alone Dedicated Control Channel in a mobile positioning procedure in a mobile communications network.
It is still another object of the present invention to provide deallocation of a semi-permanent Direct Transfer Application Part setup and the corresponding Stand alone Dedicated Control Channel after a specified period of mobile station positioning inactivity in a mobile communications network.
The present invention is directed to a system and method for efficiently utilizing the radio interface in a mobile station positioning capable mobile communications network. After control channel allocation, a maximum or timeout period of mobile station positioning inactivity is allowed after which any semi-permanently allocated control channel may be deallocated, thereby allowing the control channel to be utilized for other Switching System/Base Station System communications.